This invention relates to electrical contacts for vacuum interrupters including, specifically, contact materials useful for, but not limited to, medium voltage interrupters operating in the range of 4-38 kilovolts.
Vacuum interrupters must meet performance requirements whose attainment is substantially dependent on the characteristics of the interrupter contact materials. These requirements are diverse and require incompatible material properties so that no single contact material appears to ideally meet all requirements. Contact material selection and formulation is thus a compromise based on obtaining characteristics balanced to best meet the diverse functional requirements.
Functional requirements of vacuum interrupters include adequate (1) continuous current carrying capability, (2) transient characteristics, (3) voltage withstand, (4) short circuit current interruption ability, (5) anti-weld properties, and (6) contact erosion resistance.
The ability to continuously carry a sufficient current without excessive heat rise requires that the series resistance measured across the two abutting, i.e., closed, contacts be extremely low. Thus, electrode contacts must comprise a highly conductive material and further must have contact surfaces that continue to make a low resistance contact with the abutting electrode contact. Copper bismuth, for example, has excellent current carrying capabilities.
Transient characteristics, i.e., the phenomena occurring upon contact opening, includes the ability to withstand chopping. Chopping is an undesirable phenomena characterized by the immediate extinction of an arc produced upon contact opening. Immediate extinction of the arc, i.e., before "current zero" of the a.c. source, occurs primarily under conditions of low load current, e.g., 5-50 amps. Such chopping can result in transient overvoltages and reignition of the arc after "current zero". Chopping is reduced by contact materials that produce vapor emissions responsive to the arc to sustain the arc from the time of current interruption to current zero. For example, in copper bismuth contacts, bismuth emits vapors at relatively low arc temperature to sustain the arc until current zero.
The vacuum interrupter should also have adequate voltage withstand. This means that there should be no current conduction or arcing across open or even partially open contacts as a result of the voltages applied across the contacts. Such conduction or reignition should be avoided.
Interruption ability is the ability of the interrupter to open and to "clear a fault" under short circuit conditions. The interruption rating, i.e., the maximum current at which the interrupter can extinguish the arc and clear the fault, is adversely affected by the release and presence of excessive vapor emissions during heavy arcing. Certain contact materials that emit electrons or excessive gas vapors under severe arc conditions are thus undesirable.
Interrupter contacts also must have adequate antiweld properties to assure that contacts can be reopened when so commanded. Certain materials, such as pure copper, tend to weld or stick when closed onto a fault current. Contacts of such materials could fail to be opened or require excessive force to open. Contact materials containing bismuth, a non-refractory brittle material having a low melting temperature, provide excellent anti-weld characteristics.
A vast variety of contact materials has been disclosed for achieving various contact performance and processing requirements. Many include mixtures of a highly electrically and thermally conductive, non-refractive metal, such as copper, with a semi-refractory metal of low ductility. Refractory metals having a very high boiling point, such as tungsten, are considered to be undesirable for short circuit current interruption. This is based on the theory that the temperature of hot spots produced on the contact surface by arcing is below their boiling point temperature but is sufficiently high to emit free electrons. Such emission of electrons impedes arc quenching during fault current interruption and thus impairs interruption capability.
It has thus been proposed to mix copper with semi-refractive metal elements having melting and boiling temperatures of an intermediate range, such as chromium, cobalt, iron, nickel, titanium, vanadium and zirconium. Copper chromium has been cited as having suitable interruption and contact erosion characteristics and thus has been selected for practical application.
Such copper-chrome compositions have been prepared by an impregnation process. The semi-refractory metal element is pressed and sintered in vacuum or in a reducing atmosphere. The resulting matrix is then impregnated with a high conductivity material such as copper in vacuum such that the infiltrated metal is retained in essentially non-alloyed form. In such impregnated sintered compositions, it is difficult to obtain the high percentage of conducting material desired. Therefore, such compositions have instead been prepared by a press and sinter process. The copper and semi-refractive metal powders are admixed, cold-pressed and sintered in vacuum or a reducing atmosphere at temperatures below the melting point of constituents. Products of high density, e.g., in excess of 90%, can be obtained by repetitive cold-pressing and sintering or by hot densification. Despite their low porosity, such materials can still emit substantial gas when subjected to high temperatures. However, during contact operation, the gettering action of the semi-refractive metal, i.e., chromium, is believed to assist in arc quenching for the purpose of fault clearing. Efforts, e.g., particle size selection, have been directed to improving other characteristics of copper-chrome compositions, such as improving their tensile or anti-weld characteristics.